1. Field of the Invention
The present invention relates generally to capacitive discharge ignition circuits to ignite the fuel mixture in a combustion chamber and specifically relates to such capacitive discharge ignition used with outboard motors to ignite the fuel mixture in the combustion chamber. Capacitive discharge ignition requires a charging voltage to charge a capacitor to approximately 300 volts. An electronic circuit then triggers a switch to discharge the capacitor to an ignition coil that generates a high voltage spark.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
It is well known in the prior art to recover energy from an inductive load that is created when the inductive load is switched off. In U.S. Pat. No. 4,318,155 residual magnetism in the engageable members of an electromagnetic clutch is relieved following de-energization of the clutch coil by charging a capacitor with the inductive energy stored in the coil at de-energization thereof and subsequently discharging the capacitor into the coil in a direction opposite to that applied to the coil during energization.
In U.S. Pat. No. 4,974,114 an energy recovery system is disclosed in which, when drive power is removed from the inductive load by switching off the drive transistors, a reverse EMF is established in the inductive load and as the inductive load magnetic field collapses, a reverse voltage is developed that is higher than the applied voltage to forward bias diodes. Thus, current flows from the inductive load through diodes to the storage capacitor for use with the next charging cycle of the transducer.
In the operation of some outboard internal combustion motors, electrical requirements may begin to exceed available power because of limited space on the stator. The alternator coils must share space with charge coils for CD (capacitive discharge) ignition. By using only alternator coils on the stator, electrical power could increase significantly. Without charge coils, there must be an alternative way to provide the high voltage needed for CD ignition.
Inasmuch as the fuel injector coil already exists as one of the elements of the internal combustion outboard motor and inasmuch as it generates a flyback voltage when the voltage is removed therefrom, it would be convenient to use the existing injector coil to provide the voltage for the ignition system. In one such fuel injector, a large flyback voltage is generated when current flow to the solenoid is cut off. Solenoids typically use a flyback diode to control this voltage but in this case it cannot be used because a flyback diode will not allow the injector to run at its required maximum frequency. Therefore, the electronic switching device that turns the injector ON and OFF must be capable of dissipating the energy that is generated when the coil is shut off. This is usually done with an electronic switch containing an integral zener diode. In one such transistor switch, the voltage at which the integral zener diode begins conducting is between 350 and 400 volts. So when the fuel injector is shut off, the flyback voltage climbs to about 350 volts before the integral zener diode turns ON and shunts to ground.
It would be advantageous to use that flyback voltage to charge the capacitor that is used for capacitive discharge ignition.